Cloak Of Invisibility

If they thought anyone was watching, they wouldn’t do things like spend hours walking around the site staring at the mud, occasionally kicking stones to see if they’re bolted down or not. (To date, he hasn’t found any bolted down — Ed)

What they don’t appear to know is that people in the surrounding buildings have a far superior Cloak Of Invisibility(tm), resulting in us being able to watch them, without them ever realising it.